Biology Sikshya

• They may be aquatic or moist terrestrial habitat.

• They may be free living or parasitic.

• Triploblastic animals i.e., body derived from 3 germ layer (ectoderm, mesoderm and endoderm).

• Body is elongated, cylindrical, vermiform (worm like) and with or without division into regions but unsegmented.

• Body is bilaterally symmetrical.

• Body is unsegmented.

• Organ-system grade of body organization.

• A pseudocoelom (not lined by mesodermal epithellium) is present. 

• Locomotion is basically wriggling or thrashing with the longitudinal muscles on one side contracting and another side expanding.

• Body wall is externally covered by cuticle of scleroprotien and with a syncytial epidermis.

• Cuticle is moulted through out the life.

• Musculature of the body contains only the longitudinal muscles.

• Cilia mostly absent.

• Complete digestive system is present with definite mouth and anus. Anterior and posterior part of alimentary are termed as stomodaeum and proctodaeum respectively. Tract is non muscular.

• Respiration occurs through general body surface.

• Excretory system consists of a protonephridia. Some forms have gland cells or canals or both. 

• Circulatory system is absent and if present, is poorly developed.

• Nervous system includes a cerebral ganglion. Anterior and posterior nerve arises from it.

• Sexual reproduction takes place.

• Development is indirect. Fertilization is internal.

• Most of them are dioecious.

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